This is a study of non-thermal food preservation, but it cites two studies that measured ascorbate loss from pasteurisation. I can't cite the studies directly because I can't find them online. According to these authors, they reported losses of 7.0-15.0% and 18.0% after 95°C for 15s. This is high compared to the pulsed electrical field method.
This study compared ascorbate loss during storage after PEF or thermal treatment. While no difference was observed when stored at 22°C, PEF treated orange juice maintained significantly higher ascorbate concentration than thermally treated orange juice when stored at 4°C. The authors attribute this to the "higher processing temperature".
They also say (and reference other studies):
Ascorbic acid is a typically heat sensitive nutrient. High temperatures during processing and storage cause loss of ascorbic acid.